1. Technical Field
This invention relates in general to communication systems and information and data processing systems, and more particularly, to techniques for evaluating data link performance by measuring the quality of data received through the link.
2. Description of the Prior Art
By way of explanation, a digital communication link, such as a high speed fiber optic data communication link, typically consists of a transmitter, a line (for example, an optical fiber) and a receiver. In a fiber optic communication link, the transmitter converts electrical data signals into optical pulses and couples them into the optical fiber. The optical signal is typically attenuated and distorted as it passes through the optical fiber. At the receiving end, a photodetector converts optical signals into electrical data pulses and, conventionally, a link status monitor measures the amplitude of the photodetector output current.
In one status monitoring technique, if the average photocurrent amplitude exceeds a predefined level, an assumption is made that the arriving data is valid. Conversely, if the average photocurrent is less than the predetermined level, it is assumed that there is a broken fiber or other failed link component, e.g., a failed transmitter module (see Cooke et al., "Integrated Circuits for a Two Hundred - Mbit/s Fiber Optic Link," IEEE J. Solid State Circuits, Vol. SC-21, No. 6, December, 1986). Another known method of monitoring link status is based on peak detecting of the analog signal from the receiver (see Uda et al., "Integrated Optical Fiber Data Link Transmitter/Receiver for Local Area Network Application," Fiber and Integrated Optics, Vol. 5, Nov. 3, 1985).
Both of the above-identified analog-type link status monitoring techniques indicate whether the optical power level (i.e., amplitude) is satisfactory, but neither provides information on possible subtle degradation in data signal quality. In the case of degradation, the data signal still passes through the digital data link, but data jitter may exceed or be close to exceeding predefined specification requirements for the link. This data timing jitter may result in the data bit error rate becoming unacceptably high. Such a situation could occur, for example, if the optical fiber became damaged or the optical connectors became obstructed.
Pursuant to the present invention, timing jitter, which is used as a measure of circuit degradation, is measured as a deviation from the ideal time position of the received signal. Three types of jitter are normally identified in fiber optic communication links, i.e., duty cycle distortion, data dependent jitter and random jitter (see "American National Standard for Information Systems -- Appendix E -- System Jitter Allocations," Rev. 4, April, 1989). Duty cycle distortion, also called pulse width distortion, is often caused by differences in propagation delay between low-to-high and high-to-low data transitions. Data dependent jitter is caused by imperfections in gain and phase characteristics of the transmission channel. It is also partially due to inter data symbol interference from a bandwidth limited non-Nyquist channel. Data dependent jitter also includes effects of non-dc balanced coding. Random jitter is caused primarily by thermal noise and normally its average is zero. The major contributor of random jitter is the optical receiver. In summary, applicants are unaware of any pre-existing digital technique which monitors data timing jitter as a measure of the quality of the data link over which the data has been received.
In view of the above, there exists a need in the art for a data link performance monitor capable of evaluating data link performance degradation before an actual fail occurs and, in particular, for such a monitor/diagnostic tool which digitally evaluates the received data signal quality (i.e., by analyzing data timing jitter) as an indication of data link performance.